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President Donald Trump touched down Wednesday in Iraq, where he greeted U.S. troops in his first visit to a conflict zone as commander in chief, defended his decision to withdraw forces from neighboring Syria and declared that the Islamic State is "very nearly defeated".

The president's visit to Al Asad Air Base, which was shrouded in secrecy, follows months of public pressure for him to spend time with troops deployed to conflicts in the Middle East and Central Asia and punctuates the biggest week of turmoil the Pentagon has faced during his presidency.

Trump, who was accompanied by first lady Melania Trump, sounded a triumphant note as he addressed US service members on the day after Christmas.

The president said, "We're no longer the suckers, folks," and he warned that he was committed to drawing down the U.S. military presence around the globe - even in instances in which his administration's experts object.

"The United States cannot continue to be the policeman of the world," Trump said. "It's not fair when the burden is all on us, the United States."

After a briefing with military and diplomatic leaders on the ground, Trump said he would deny any request from generals to extend the operation in Syria, where roughly 2000 US troops are serving."

They said again, recently, can we have more time?" Trump said of U.S. generals. "I said, 'Nope.' You can't have any more time. You've had enough time. We've knocked them out. We've knocked them silly."

President Donald Trump kisses first lady Melania Trump during a hanger rally at Al Asad Air Base, Iraq.Photo / AP

"I think a lot of people are going to come around to my way of thinking," Trump said, according to Reuters. "It's time for us to start using our head."

Trump, who spent the holiday at the White House amid a partial federal government shutdown, departed Andrews Air Force Base near Washington aboard Air Force One at 12.06am Wednesday, flying in the dark of night in an attempt to preserve operational security.

After an 11-hour flight, the president and first lady landed at Al Asad Air Base at 7.16pm local time.

Trump was scheduled to meet with Iraqi Prime Minister Adel Abdul-Mahdi, but the meeting was canceled for undisclosed reasons.

Trump travelled with a small group of journalists, who were ordered for security reasons not to report that he was in Iraq until he had finished delivering his remarks to the troops, roughly three hours into his visit to the base.

Trump told reporters that Iraq is "a place I have been talking about for many years - many, many years. I was talking about it as a civilian."

The president added, "I want to come and pay my respects, most importantly, to the great soldiers, great troopers we have here."

Although Trump initially supported the invasion of Iraq in 2003, he later criticised the effort. He was also critical of President Barack Obama's withdrawal from Iraq in 2011, saying the move created the Islamic State.

Trump's unannounced visit on the day after Christmas continues a holiday tradition followed by previous presidents. Vice President Pence visited Afghanistan on December 22, 2017, to address troops and meet with commanders at Bagram Airfield near Kabul.

US President Donald Trump visits with members of the military at a dining hall at Al Asad Air Base, Iraq. Photo / AP

Al Asad Air Base, which is located northwest of Ramadi, is a city that the Islamic State captured before U.S.-backed Iraqi forces retook the area.

Trump said he considered the safety risks in making his first trip to a war zone.

"I had concerns about the institution of the presidency," he told reporters.

"Not for myself personally. I had concerns for the first lady, I will tell you. But if you would have seen what we had to go through in the darkened plane with all windows closed, with no light anywhere. Pitch black. I've been on many airplanes. All types and shapes and sizes."

"So did I have a concern?" he continued. "Yes, I had a concern."

Obama visited Afghanistan four times as president, most recently in 2014, and made a trip to Iraq shortly after his 2009 inauguration.

President George W. Bush made a surprise Thanksgiving visit to troops in Iraq after the invasion in 2003 and traveled back three additional times as commander in chief.

President Bill Clinton visited troops in Bosnia in 1996 and spent Thanksgiving with troops in Kosovo in 1999, while George H.W. Bush spent Thanksgiving with service members in Saudi Arabia during Operation Desert Shield in 1990 and New Year's Day with troops in Somalia in 1993.

On Wednesday in Iraq, the Trumps mingled with uniformed service members at a base dining facility that was decorated for Christmas with foil balls, twinkling lights and snowmen made of stacked tires painted white.

The president stopped to sign several "Make America Great Again" hats that service members brought, and at one point he autographed an embroidered patch that read "TRUMP 2020."

Trump stopped to talk football with a soldier from Georgia, discussing the Atlanta Falcons' recent losses, and he posed to take selfies with many troops.

Kyu Lee said that after telling Trump that he was the chaplain for SEAL Team Five, the president responded: "Hey, in that case, let's take a picture."

President Trump and the First Lady traveled to Iraq late on Christmas night to visit with our troops and Senior Military leadership to thank them for their service, their success, and their sacrifice and to wish them a Merry Christmas. pic.twitter.com/s2hntnRwpw

Trump's visit comes at a moment of pitched tension with the Defence Department that belies the feel-good atmosphere on base in Iraq.

Mattis tendered his resignation Thursday over Trump's decision to withdraw troops from Syria and draw down numbers in Afghanistan in addition to other differences. Although Mattis planned to leave in February, Trump on Sunday dismissed him early, demanding the retired Marine Corps general leave the Pentagon by year end.

Mattis and other top advisers had cautioned Trump not to order a precipitous withdrawal from Syria, where roughly 2000 US troops enabled a Kurdish-led militia to roll back the self-declared caliphate of the Islamic State.

Despite declaring victory over the Islamic State, Trump later tweeted that Turkish President Recep Tayyip Erdogan "very strongly informed me that he will eradicate whatever is left of ISIS in Syria".

Turkey views the Kurdish-led force U.S. troops have relied on to roll back the Islamic State in Syria as enemies, and Pentagon officials worry Turkish forces will massacre the Kurds while failing to prevent a resurgence of ISIS.

Trump hasn't said anything in the past week about the U.S. presence in Iraq, where some 5000 American troops remain, assisting Iraqi government forces and Kurdish Peshmerga fighters in the north in the effort to secure the nation from the threat of the Islamic State. US forces reentered Iraq in 2014 after the Islamic State established a self-declared caliphate in Syria and swept through Iraq, reaching the outskirts of the capital, Baghdad.

Although Trump has previously addressed US troops stationed overseas, including in Italy, Japan and South Korea, he has drawn criticism for not yet visiting those deployed to combat zones.

Trump has long been critical of the war in Afghanistan, which was born of the September 11, 2001, terrorist attacks and has stretched on for more than 17 years, becoming America's longest war.

The United States is currently engaged in peace talks with the hope of ending a long-running stalemate between a resurgent Taliban and the Afghan government forces that Washington backs.

But Trump has directed the Defence Department to withdraw nearly half of the more than 14,000 troops deployed to Afghanistan, US officials said last week, despite warnings from many of the president's senior advisers and military officials that the move would invite chaos and terrorism.

Trump has described the US involvement in Afghanistan as a "complete waste" and vowed to bring home American troops. Top U.S. officials have warned that absent a peace deal with the Taliban, Afghanistan could collapse and descend into further turmoil after a precipitous US withdrawal.

The Afghanistan withdrawal order came alongside Trump's abrupt announcement last week that he would pull all 2000 US troops from Syria, a decision made against the counsel of his top advisers and without warning allies in the fight there against the Islamic State.

Mattis resigned in the wake of the Syria decision, writing in his resignation letter that he and Trump disagree in their worldview and that the president's policies threaten the alliance structure the United States helped construct in the aftermath of World War II.

Trump was furious with media coverage of Mattis' resignation, aides said, and especially the notion of the defense secretary as a human guardrail protecting the country from the president's impulses. In retaliation, Trump moved Sunday to oust him ahead of schedule, directing Secretary of State Mike Pompeo to deliver the news to Mattis because the president was not on speaking terms with his outgoing defence secretary.